Fuel injector

ABSTRACT

A fuel injector for use in an injector arrangement including a pumping plunger, a pump chamber and a spill valve arrangement controlling communication between the pump chamber and a low pressure fuel reservoir. The fuel injector comprises a valve needle which is engageable with a seating to control fuel delivery through a fuel injector outlet opening, a first control chamber being arranged such that the fuel pressure therein acts on a surface associated with the valve needle and a second control chamber being arranged such that fuel pressure therein acts on a surface associated with the spill valve arrangement. The injector further comprises a piezoelectric actuator arrangement including a piezoelectric element having first and second ends, the first end being associated with the spill valve arrangement and the second end being associated with the valve needle such that, in use, operation of the spill valve arrangement and movement of the valve needle is controlled by controlling the energisation level of the piezoelectric element.

TECHNICAL FIELD

This invention relates to a fuel injector for use in supplying fuel,under pressure, to the cylinders of an internal combustion engine.

BACKGROUND OF THE INVENTION

A known fuel injector arrangement comprises a plunger reciprocablewithin a plunger bore provided in a housing to pressurise fuel locatedwithin a pump chamber defined by the plunger bore. The plunger borecommunicates with a fuel pressure actuated injector such that, once thefuel pressure within the plunger bore exceeds a predetermined level, theinjector opens and, thus, fuel injection commences.

SUMMARY OF THE INVENTION

In order to permit independent control of the injection pressure and thetiming of injection, it is known to provide a spill valve whichcommunicates with the pump chamber, and an injection control valve whichcontrols fuel pressure within a control chamber defined, in part, by asurface associated with a valve needle of the injector to controlmovement of the needle. In use, the spill valve remains open duringinitial inward movement of the plunger. Subsequently, the spill valve isclosed, further inward movement of the plunger pressurising the fuelwithin the pump chamber. When injection is to commence, the injectioncontrol valve is actuated to connect the control chamber to a lowpressure drain thus permitting movement of the needle away from itsseating to commence fuel injection.

European Patent Application No 0840003 describes a fuel injector of theaforementioned type, including a spill valve arrangement, which iscontrolled by means of a first actuator, and an injection control valve,which is controlled by means of a second actuator. As the fuel injectorrequires two actuator arrangements to independently control the spillvalve and the injection control valve, the cost of the injector is high,particularly if piezoelectric actuators are employed.

European Patent Application No 0905719 describes a method ofindependently controlling the spill valve and the injection controlvalve of a fuel injector by means of a single, electromagnetic actuatorarrangement. When it is desired to pressurise fuel within the pumpchamber, the spill valve is closed by energising the actuator to afirst, relatively low energisation level, during which stage ofoperation the injection control valve remains closed to low pressure. Inorder to commence fuel injection, the actuator is energised to a second,higher energisation level to open the injection control valve to lowpressure. When the injection control valve is open to low pressure, thefuel injector valve needle is moved inwardly to permit fuel injectionthrough outlet openings provided in the fuel injector nozzle body.

In order to permit full filling of the pump chamber and to minimise thevolume of the plunger bore and maximise the peak fuel pressure, it isdesirable to locate the spill valve in close proximity to the plunger.In addition, it is desirable to locate the injection control valve inclose proximity to the valve needle as this improves the response of thevalve needle. However, it is difficult to satisfy both of theserequirements using the fuel injector arrangements describedhereinbefore.

It is an object of the invention to provide a fuel injector whichalleviates this problem.

According to the present invention there is provided a fuel injector foruse in an injector arrangement including a pumping plunger and a pumpchamber and a spill valve arrangement controlling communication betweenthe pump chamber and a low pressure fuel reservoir, the fuel injectorcomprising a valve needle engageable with a seating to control fueldelivery through a fuel injector outlet opening, a first control chamberbeing arranged such that the fuel pressure therein acts on a surfaceassociated with the valve needle and a second control chamber beingarranged such that fuel pressure therein acts on a surface associatedwith the spill valve arrangement, and a piezoelectric actuatorarrangement including a piezoelectric element having first and secondends, the first end being associated with the spill valve arrangementand the second end being associated with the valve needle such that, inuse, operation of the spill valve arrangement and movement of the valveneedle is controlled by controlling the energisation level of thepiezoelectric element.

The invention provides the advantage that only a single piezoelectricactuator arrangement is required. This reduces the cost of the fuelinjector. Additionally, the spill valve and the control chamberassociated with the valve needle can both be located in close proximityto the piezoelectric actuator arrangement. Thus, the response of thevalve needle is improved.

The fuel injector conveniently includes first and second piston members,the first piston member being moveable with the first end of thepiezoelectric element and the second piston member being movable withthe second end of the piezoelectric element, movement of the first andsecond piston members being controlled by varying the energisation levelof the piezoelectric element.

Conveniently, the first and second piston members are slidable withinfirst and second bores respectively. The first bore may define, in part,the first control chamber and the second bore may define, in part, thesecond control chamber. Movement of the first piston member within thefirst bore varies the volume of the first control chamber and movementof the second piston member within the second bore varies the volume ofthe second control chamber.

The first piston member may have a smaller diameter than the secondpiston member such that de-energisation of the piezoelectric element toa first energisation level causes movement of the second piston memberto close the spill valve arrangement and further de-energisation of thepiezoelectric element to a second, lower energisation level causesmovement of the first piston member to move the valve needle away fromthe seating.

Alternatively, the first piston member may have a diameter greater thanthe diameter of the second piston member such that energisation of thepiezoelectric element to a first energisation level causes movement ofthe second piston member to close the spill valve arrangement andfurther energisation of the piezoelectric element to a second, higherenergisation level causes movement of the first piston member to movethe valve needle away from the seating.

In one embodiment of the invention, the fuel injector may furthercomprise an injection control valve arrangement including a controlvalve member and a third control chamber for fuel, a surface of thecontrol valve member being exposed to fuel pressure within the thirdcontrol chamber, the control valve member being engageable with afurther seating to control fuel pressure within the first controlchamber.

Conveniently, the spill valve arrangement comprises a spill valve memberwhich is slidable within a further bore defining, in part, an additionalchamber. The spill valve member may be engageable with a further seatingto control communication between the additional chamber and a lowpressure fuel reservoir. Alternatively, the additional chamber maycommunicate directly with a low pressure fuel reservoir.

In an alternative embodiment, the first control chamber may be arrangedto communicate directly, by means of a first restricted flow path, witha supply passage for supplying fuel under high pressure to the injector.

The second control chamber may be arranged to communicate directly, bymeans of a second restricted flow path, with a supply passage forsupplying fuel under high pressure to the injector.

The piezoelectric actuator arrangement may include a singlepiezoelectric element or may include a stack of piezoelectric elements.

The valve needle may be of the inwardly or outwardly opening type.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example only, withreference to the accompanying drawings, in which:

FIG. 1 is a cross-sectional view of a fuel injector in accordance withan embodiment of the present invention;

FIG. 2 is a schematic view of the fuel injector in FIG. 1;

FIG. 3 is a cross-sectional view of an alternative embodiment; and

FIG. 4 is a schematic view of the fuel injector in FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIGS. 1 and 2, the fuel injector includes a valve needle 12which is reciprocable within a bore 11 provided in a nozzle body 10. Thevalve needle 12 is engageable with a seating defined by the bore 11 tocontrol fuel delivery through one or more outlet openings (not shown)provided in the nozzle body 10. The bore provided in the nozzle body 10includes a region of enlarged diameter which defines an annular chamber14, fuel being supplied to the annular chamber 14 by means of a supplypassage 16, in use. The valve needle 12 is provided with one or morethrust surfaces 12 a which are exposed to fuel pressure within theannular chamber 14.

The upper end of the nozzle body 10 abuts a distance piece 18 which isprovided with a bore 20 of stepped form, a first piston member 22 beingslidable within an upper region of the bore 20. The bore 20 includes aregion having a diameter greater than that of the piston member 22, andincludes a region of reduced diameter which communicates with a recess26 provided in the upper end face of the nozzle body 10, the recess 26,the lower end face of the piston member 22 and the bore 20 togetherdefining a first control chamber 25 for fuel. In use, fuel pressurewithin the control chamber 25 acts on the upper end face of the valveneedle 12 and serves to urge the valve needle 12 in a downwardsdirection in the illustration shown against the seating defined by thebore 11. As indicated in FIG. 2, the upper end face of the valve needle12 and the lower end face of the distance piece 18 together define aclearance gap, D1, which serves to limit the extent of movement of thevalve needle 12 within the bore 11. Additionally, the lower end face ofthe first piston member 22 and an upwardly directed interal surface ofthe control chamber 25 together define a second clearance gap, D2, whichserves to limit the extent of movement of the piston member 22 withinthe bore 20, the bore 20 being provided with a step which defines a stopfor the piston member 22 when the piston member 22 has moved through adistance equal to D2 in a downward direction, in the illustration shown.

The distance piece 18 is provided with a drilling which forms part ofthe supply passage 16 for fuel. As shown in FIG. 2, the supply passage16 communicates, by means of a restricted passage 28, with the firstcontrol chamber 25. The control chamber 25 is also supplied with highpressure fuel through leakage from the bore 11. Alternatively, thecontrol chamber 25 may communicate with the supply passage 16 throughleakage alone, rather than by providing the restricted passage 28.

The distance piece 18 abuts a housing 30 within which a piezoelectricactuator arrangement is arranged, the piezoelectric actuator arrangementincluding a piezoelectric stack 32 which is housed within a chamber 33defined within the housing 30. In use, the energisation level, and hencethe axial length, of the piezoelectric stack 32 may be varied by varyingthe voltage applied to the stack 32. One end of the piezoelectric stack32 is in connection with the piston member 22, the other end of thepiezoelectric stack 32 being in connection with a second piston member34 which is reciprocable within a bore 36 provided in the housing 30.The piezoelectric stack is housed within a chamber which is connected,by means of a passage 37, to a low pressure fuel reservoir or drain.Thus, in use, any fuel under high pressure which leaks into the chamberhousing the piezoelectric stack 32 is able to escape to low pressurethrough the passage 37.

At its end remote from the distance piece 18, the housing 30 abuts afurther housing 38 for a spill valve arrangement, the spill valvearrangement including a spill valve member 40 including a region 40 a ofenlarged diameter, a region 40 b of reduced diameter and an intermediateregion 40 c of further reduced diameter. The spill valve member 40 isslidable within a through bore 39 of stepped form provided in thehousing 38. The diameter of the spill valve member region 40 b issubstantially the same as the diameter of the adjacent part of the bore39 such that movement of the spill valve member 40 is guided within thebore. The upper surface of the enlarged region 40 a of the spill valvemember 40 and the step in the bore 39 together defining a clearance gap,D3, which serves to limit the extent of movement of the spill valvemember 40 within the bore 39. A surface of the spill valve member 40defined between the intermediate region 40 c and the region 40 b isengageable with a valve seating defined by the bore 39 to controlcommunication between the supply passage 16 and a chamber 42 defined bya region of the bore 39. The chamber 42 communicates with the lowpressure fuel reservoir by means of a flow passage 43.

It will be appreciated that the enlarged region 40 a and theintermediate region 40 c of the spill valve member 40, or theintermediate region 40 c and the region 40 b of the spill valve member40, may be integrally formed. Alternatively, the regions 40 a, 40 b and40 c of the spill valve member 40 may be separate components which areheld together by forces due to fuel pressure or by means of biasingsprings.

The lower end face of the enlarged region 40 a of the spill valve member40, the bore 36 provided in the housing 30 and the upper end face of thepiston member 34 together define a second control chamber 44 for fuel,the chamber 44 being in communication with the supply passage 16 bymeans of a restricted passage 46, as indicated in FIG. 2. The diameterof the first piston member 22 is slightly smaller than the diameter ofthe second piston member 34. Thus, the effective surface area of thepiston member 22 exposed to fuel pressure within the chamber 25 isslightly smaller than the effective surface area of the second pistonmember 34 exposed to fuel pressure within the chamber 44.

The housing 38 abuts, at its end remote from the housing 30, a furtherhousing 47, the nozzle body 10, the distance piece 18 and the housings30, 38, 47 being received within a cap nut 51. The housing 47 isprovided with a bore 48 which defines a pump chamber 52 for fuel. Apumping plunger 50 is reciprocable within the bore 48 to vary the volumeof the pump chamber 52, the pumping plunger 50 being cooperable with acam arrangement (not shown) which is carried by a drive shaft such that,upon rotation of the drive shaft, reciprocating motion is transmitted tothe pumping plunger 50. The bore 48 communicates with the low pressurefuel reservoir by means of a flow passage 49 provided in the housing 47.

The pump chamber 52 communicates with the supply passage 16 such that,in use, during outward movement of the pumping plunger 50 within thebore 48 and with the spill valve member 40 in its open position, fuel isdrawn into the pump chamber 52, subsequent inward movement of thepumping plunger 50 within the bore 48 causing fuel to be expelled fromthe pump chamber 52, past the valve seating, into the chamber 42 and tothe low pressure fuel reservoir. If the spill valve member 40 is movedto its closed position, fuel within the pump chamber 52 is unable toflow to low pressure such that inward movement of the pumping plunger 50within the bore 48 causes pressurisation of fuel within the pump chamber52, high pressure fuel therefore being supplied through the supplypassage 16 to the downstream parts of the fuel injector.

In use, starting from a position in which the piezoelectric stack 32 isenergised to a higher energisation level, and with the spill valvemember 40 in its open position, fuel within the pump chamber 52 is ableto escape to low pressure as the pumping plunger 50 reciprocates withinthe bore 48. Fuel within the pump chamber is able to flow through thesupply passage 16 into the annular chamber 14, into the first controlchamber 25 through the passage 28, and through leakage from the bore 11,and into the second control chamber 44 through the passage 46. Fuelpressure within the chamber 25 acts on the upper end face of the valveneedle 12 and, as the effective area of the end face of the valve needle12 exposed to fuel pressure within the chamber 25 is greater than theeffective area of the thrust surface 12 a exposed to fuel pressurewithin the chamber 14, the valve needle 12 is urged against the seatingto prevent fuel delivery through the outlet openings provided in thenozzle body 10. Thus, fuel injection does not take place. It will beappreciated that the valve needle 12 may also be urged against itsseating by means of a compression spring (not shown) in a conventionalmanner. Fuel within the passage 43 and in the control chambers 25, 44 isat a higher pressure than fuel within the passage 37 and the chamber 33and, as the piston member 34 has a larger effective area exposed to fuelpressure than the piston member 22, the net force on the piezoelectricstack 32 is therefore in a downwards direction causing the piston member22 to be urged in a downwards direction to close the clearance gap D2.

When fuel pressurisation is to be commenced, the actuator is partlyde-energised to a first, reduced energisation level causing the lengthof the piezoelectric stack 32 to contract to a first, reduced length. Asthe diameter of the piston member 34 is slightly greater than thediameter of the piston member 22, the effective area of the pistonmember 34 exposed to fuel pressure within the chamber 44 is greater thanthe effective area of the piston member 22 exposed to fuel pressurewithin the chamber 25. Thus, as a result of the contraction in length ofthe piezoelectric stack 32 to the first, reduced length, the secondpiston member 34 will be caused to move by a greater amount than thepiston member 22 which should remain against its stop defined by thebore 20. As a result, the volume of the chamber 44 is increased suchthat fuel pressure within the chamber 44 reduces, fuel only beingsupplied to the chamber 44 at a restricted rate through the restrictedpassage 46. The force applied to the end face of the enlarged region 40b of the spill valve member 40 is therefore reduced, fuel pressurewithin the pump chamber 52 acting on the end face of the region 40 b ofthe spill valve member 40 so as to urge the spill valve member 40against its seating to close communication between the supply passage 16and the low pressure fuel reservoir. Thus, during continued inwardmovement of the pumping plunger 50 within the bore 48, fuel pressurewithin the pump chamber 52 increases and fuel under high pressure isdelivered through the supply passage 16 to the downstream parts of thefuel injector.

When fuel injection is to be commenced, the piezoelectric actuator isfurther de-energised to a second, further reduced energisation level,the length of the piezoelectric stack 32 thereby contracting to afurther reduced, second length. As the spill valve member 40 is seated,any further contraction of the length of the piezoelectric stack 32gives rise to a relative reduction in fuel pressure within the controlchamber 44 to permit the piston member 22 to move away from its stopdefined by the bore 20. Thus, as fuel is only able to flow into thechamber 25 at a restricted rate through the restricted passage 28, whenthe piezoelectric stack 32 is further contracted, the first pistonmember 22 is moved in an inwards direction in the illustration shown inFIG. 1, thereby increasing the volume of the chamber 25. The forceapplied to the upper end face of the valve needle 12 is thereby reduced,fuel pressure within the chamber 14 acting on the thrust surface 12 a ofthe valve needle 12 and serving to urge the valve needle 12 in anupwards direction away from the seating to permit fuel delivery throughthe outlet openings provided in the nozzle body 10. Fuel is thereforeinjected into the engine cylinder or other combustion space. Movement ofthe valve needle 12 away from its seating is limited by the clearancegap, D1, defined between the upper end face of the valve needle 12 andthe distance piece 18.

In order to cease fuel injection, the actuator arrangement isre-energised to the first energisation level, thereby increasing thelength of the piezoelectric stack 32 to the first length. The pistonmember 22 is therefore caused to move downwardly within the bore 20 toreduce the volume of the chamber 25, fuel pressure within the chamber 25thereby being increased. Thus, the force applied to the upper end faceof the valve needle 12 is also increased and is sufficient to overcomethe force applied to the thrust surface 12 a of the valve needle 12 suchthat the valve needle 12 is returned to its seated position. Thus, fueldelivery through the outlet openings ceases. During this stage ofoperation, as the length of the piezoelectric stack 32 is increased, thesecond piston member 34 is moved upwardly to reduce the volume of thechamber 44. However, although fuel pressure within the chamber 44increases, the force on the enlarged region 40 a of the spill valvemember 40 is insufficient to overcome the downward force on the region40 b of the spill valve member 40 due to fuel pressure within thechamber 52. The spill valve member 40 therefore remains seated againstits seating to maintain fuel pressurisation within the pump chamber 52.

In order to cease fuel pressurisation, the piezoelectric stack isenergised to the initial, higher energisation level to increase thelength of the piezoelectric stack 32 to the initial length, the pistonmember 34 therefore being moved in an upwards direction to reduce thevolume of the chamber 44. Fuel pressure within the chamber 44 thereforeincreases, the force on the enlarged region 40 a of the spill valvemember 40 due to fuel pressure within the chamber 44 being sufficient toovercome the downward force on the region 40 b of the spill valve member40 due to fuel pressure within the chamber 52 such that the spill valvemember 40 is urged away from its seating to open communication betweenthe supply passage 16 and the low pressure fuel reservoir. Thus,continued inward movement of the pumping plunger 50 within the bore 48does not result in pressurisation of fuel.

During the fuel injecting stage, it may be preferable to de-energise thepiezoelectric actuator to an energisation level slightly less than thesecond energisation level to ensure a net flow of fuel into the chamber25 from the supply passage 16 is maintained. Thus, when fuel injectionis to be ceased, and the piezoelectric stack 32 is re-energised, closureof the valve needle 12 against its seating is assured before the pistonmember 22 reaches its stop defined by the bore 20.

By utilising both ends of the piezoelectric stack to control movement ofthe spill valve member 40 and the valve needle 12 respectively, both thespill valve and the valve needle can conveniently be located in closeproximity to the pump chamber 52 and the control chamber 25respectively. Thus, the response of the valve needle is improved.Additionally, as only one piezoelectric actuator is required, the costof the fuel injector is reduced significantly.

In the embodiments of the invention shown in FIGS. 1 and 2, the controlchambers 25, 44 communicate directly, via the restricted passages 28, 46respectively, with the supply passage 16. As shown in FIGS. 3 and 4, inan alternative embodiment the fuel injector includes an injectioncontrol valve arrangement including a control valve member 60 which isslidable within a bore 62 provided in a fuel injector housing 67. Thefuel injector includes a first piston member 22 a and a second pistonmember 34 a, the first piston member 22 a having a diameter slightlygreater than the diameter of the second piston member 34 a.

The control valve member 60 has an end face which is exposed to fuelpressure within a third control chamber 64, the control valve member 60being engageable with a seating defined by the bore 62 to controlcommunication between the supply passage 16 and the control chamber 25.Thus, the supply passage 16 does not communicate directly with thecontrol chamber 25, as is the case in the embodiment shown in FIGS. 1and 2, the control chamber 25 being connected, via a restricted passage69, to the low pressure fuel reservoir. A spring 66 is housed within thebore 62, the spring 66 serving to urge the control valve member 60 awayfrom its seating to open communication between the supply passage 16 andthe chamber 25.

The chamber 42 defined by the bore 39 also does not communicate directlywith the low pressure fuel reservoir, communication between the chamber42 and the low pressure fuel reservoir being controlled by means of thespill valve member 40, fuel being able to flow between the low pressurefuel reservoir and the chamber 42 by means of a restricted passage 65when the spill valve member 40 is lifted away from its seating. Thespill valve member 40 is urged away from the valve seating defined bythe bore 39 by means of a spring 63 housed within an end of the bore 39.Between fuel injections, the control chambers 44, 64 fill with fuelthrough leakage and, as the piston member 34 a has a smaller effectivearea exposed to fuel pressure than the piston member 22 a, and as thechamber within which the piezoelectric stack 32 is housed communicateswith the low pressure fuel reservoir, the piezoelectric stack 32 will bemoved upwardly until the piston member 22 a reaches its stop defined bya bore in the housing 67.

In use, when fuel pressurisation is to be commenced, the piezoelectricactuator is energised to a first, relatively low energisation level, toextend the length of the piezoelectric stack 32. As the effective areaof the piston member 22 a exposed to fuel pressure within the chamber 64is greater than the effective area of the piston member 34 a exposed tofuel pressure within the chamber 44, the piston member 34 a is moved inan upwards direction within the bore 36, thereby reducing the volume ofthe chamber 44. Thus, fuel pressure within the chamber 44 is increasedand is sufficient to overcome the spring force due to the spring 63, thespill valve member 40 therefore being urged against its seating to closecommunication between the pump chamber 52 and the low pressure fuelreservoir. Continued, reciprocal motion of the pumping plunger 50 withinthe bore 48 therefore causes pressurisation of fuel within the pumpchamber 52.

When fuel injection is to be commenced, the piezoelectric actuator isenergised to a second, higher energisation level to further increase thelength of the piezoelectric stack 32. As the spill valve member 40 isseated and the control chamber 44 is closed, the first piston member 22a is caused to move downwardly within the bore 20, reducing the volumeof the chamber 64 and increasing fuel pressure therein. The forceapplied to the end face of the control valve member 60 exposed to fuelpressure within the chamber 64 is therefore increased and is sufficientto overcome the force due to the spring 66, the control valve member 60thereby being urged against its seating to close communication betweenthe supply passage 16 and the control chamber 25.

Thus, during this stage of operation, fuel pressure within the controlchamber 25 is reduced, thereby reducing the force applied to the upperend face of the valve needle 12. The valve needle 12 is therefore liftedaway from its seating due to fuel pressure within the chamber 14 actingon the thrust surface 12 a. With the valve needle 12 lifted away fromthe seating fuel injection takes place through the outlet openingsprovided in the nozzle body. It will be appreciated that, although theseparate fuel injector housing components are not shown in FIG. 3, thefuel injector may comprise similar housing components to those shown inFIG. 1.

By utilising both ends of the piezoelectric stack to control movement ofthe spill valve member and the control valve member, both the spillvalve and the control valve can be located in close proximity to thepump chamber 52 and the control chamber 25 respectively. Additionally,only one piezoelectric actuator arrangement is required, therebyreducing the cost of the fuel injector.

In an alternative embodiment to that shown in FIG. 3 and 4, a surfaceassociated with an outwardly opening valve needle may be exposed to fuelpressure within the control chamber 64 such that, when the piezoelectricstack is energised to cause movement of the piston member 22 a and anincrease in fuel pressure within the chamber 64, the force applied tothe surface associated with the valve needle is also increased to movethe valve needle outwardly, thereby permitting fuel delivery into theengine cylinder or other combustion space.

It will be appreciated that the piezoelectric actuator arrangement neednot include a stack 32 of peizoelectic elements, but may include asingle piezoelectric element.

What is claimed is:
 1. A fuel injector for use in an injectorarrangement including a pumping plunger, a pump chamber and a spillvalve arrangement for controlling communication between the pump chamberand a low pressure fuel reservoir, the fuel injector comprising a valveneedle engageable with a seating to control fuel delivery through a fuelinjector outlet opening, a first control chamber being arranged suchthat the fuel pressure therein acts on a surface associated with thevalve needle and a second control chamber being arranged such that fuelpressure therein acts on a surface associated with the spill valvearrangement, and a piezoelectric actuator arrangement including apiezoelectric element having first and second ends, the first end beingassociated with the spill valve arrangement and the second end beingassociated with the valve needle such that, in use, operation of thespill valve arrangement and movement of the valve needle is controlledby controlling energisation level of the piezoelectric element.
 2. Thefuel injector as claimed in claim 1, wherein the fuel injector includesfirst and second piston members, the first piston member being moveablewith the first end of the piezoelectric element and the second pistonmember being movable with the second end of the piezoelectric element,movement of the first and second piston members being controlled byvarying the energisation level of the piezoelectric element.
 3. The fuelinjector as claimed in claim 2, wherein the first and second pistonmembers are slidable within first and second bores respectively, thefirst bore defining, in part, the first control chamber and the secondbore defining, in part, the second control chamber.
 4. The fuel injectoras claimed in claim 2, wherein the first piston member has a smallerdiameter than a diameter the second piston member such thatde-energisation of the piezoelectric element to a first energisationlevel causes movement of the second piston member to close the spillvalve arrangement so as to close communication between the pump chamberand the low pressure fuel reservoir, further de-energisation of thepiezoelectric element to a second, lower energisation level causingmovement of the first piston member to move the valve needle away fromthe seating.
 5. The fuel injector as claimed in claim 2, wherein thefirst piston member has a diameter greater than a diameter of the secondpiston member such that energisation of the piezoelectric element to afirst energisation level causes movement of the second piston member toclose the spill valve arrangement so as to close communication betweenthe pump chamber and the low pressure fuel reservoir, furtherenergisation of the piezoelectric element to a second, higherenergisation level causing movement of the first piston member to movethe valve needle away from the seating.
 6. The fuel injector as claimedin claim 1, wherein the second control chamber is arranged tocommunicate directly, by means of a second restricted flow path, with asupply passage for supplying fuel under high pressure to the injector.7. The fuel injector as claimed in claim 1, wherein the first controlchamber is arranged to communicate directly, by means of a firstrestricted flow path, with a supply passage for supplying fuel underhigh pressure to the injector.
 8. The fuel injector as claimed in claim1, further comprising an injection control valve arrangement forcontrolling communication between the first control chamber and a supplypassage for supplying fuel under high pressure to the injector.
 9. Thefuel injector as claimed in claim 8, wherein the injection control valvearrangement includes a control valve member and a third control chamberfor fuel, a surface of the control valve member being exposed to fuelpressure within the third control chamber, the control valve memberbeing engageable with a further seating to control communication betweenthe first control chamber and the supply passage.
 10. The fuel injectoras claimed in claim 1, wherein the spill valve arrangement comprises aspill valve member which is slidable within a further bore defining, inpart, an additional chamber, the spill valve member being engageablewith a further seating to control communication between the additionalchamber and a low pressure fuel reservoir.
 11. The fuel injector asclaimed in claim 1, wherein the piezoelectric actuator arrangementincludes a stack of piezoelectric elements.
 12. The fuel injector asclaimed in claim 1, the injector being of the type in which inwardmovement of the value needle within a nozzle body bore results ininjection of fuel through the outlet opening.